TtOYAL IRISH ACADEMY. 
209 
cording as the axes are equal in length or not. Some of these are so ir¬ 
regular, however, that the radial character is almost lost, and they 
might he taken for another species ; but I think specimens sufficiently 
intermediate in character exist to refute this view. There is no difference 
in the mode of the occurrence of these species,—masses of the animals 
compressed together in layers and intermingled in the beds of what 
evidently once was sandy mud. Occasionally we find scattered fans of 
0 . antiqua, or single stems of 0 . radiata, and sometimes we get the whole 
mass so compressed and confused as to render it impossible to make out 
the parts distinctly. The species are generally found apart. I possess one 
specimen a quarter of an inch thick, in which a bed of 0. radiata over- 
lies a bed of 0 . antiqua , so that the specimen exhibits the two spe¬ 
cies on its opposite sides, and I have found scattered fans of 0. antiqua 
among 0. radiata. It would appear, then, that the two species lived 
under different circumstances, and had thus remained distinct even after 
death, just as at the present day we will find two distinct species of a 
group inhabiting the same seas at different epochs, the conditions which 
fit it for the existence of the one not being compatible with the well¬ 
being of the other. There is a form of 0. radiada (?) which might be 
easily mistaken, on hasty examination, for 0 . antiqua, a mistake I have 
seen occurring even in museum collections. It appears to be formed 
by the overlapping of a number of the tufts of 0 . radiata in an alternate 
manner; but the absence of the rachis distinguishes it at once. Sometimes 
we find the same form much drawn out and elongated. This, sometimes 
at least, is due to the distortion dependent on cleavage; but in other 
specimens itis a genuine character dependent on the mode of growth of the 
Polypidom itself, a form of development familiar to every zoophy tologist. 
The cells in which these animals dwelt appear to have been in 0. antiqua 
biserial and alternate; in some specimens of 0. radiata a similar arrange¬ 
ment is remarkably evident, but in others belonging to the second form 
described, and in which the termination of each of the axes is enlarged, this 
arrangement is not so evident. Springing from the axils of these cells in 
one or two of my specimens of 0 . radiata are somewhat elliptical bodies, 
which, although not strongly marked, are, I think, sufficiently so to satisfy 
me in considering them as oviferous capsules; I have not detected these 
organs in 0. antiqua. One form of 0. radiata presents the appearance 
of long, unbranched axes. This is an appearance also well known among 
recent Hydrozoa. 
These are the more important points I have been enabled to make 
out regarding this genus as established by Forbes, and, taking into con¬ 
sideration the close approximation, almost amounting to identity in form, 
which exists between these and the Sertularia, their mode of growth, as 
far as we can judge, and place of occurrence, being so similar,—so much 
so that a superficial examination would lead one to pronounce them iden¬ 
tical,—I think we are justified in considering these rather Sertularian 
Polypidoms than Polyzoan Csensecia,—-a view of the genus which, I am 
convinced, further research among living forms will eventually prove 
to be the correct one. 
